As a result of recent developments in electronics, there is a rapidly increasing demand for semiconductor devices such as memories and integrated circuits.
Such semiconductor devices are manufactured by doping semiconductor substrates of a very high purity with impurities to impart electrical properties thereto, by stacking together semiconductor substrates with minute circuit patterns formed thereon, etc.
In order to avoid the influences of dust in the air, etc., such operations must be conducted in a chamber in a high vacuum state. To evacuate this chamber, a vacuum pump is generally used; in particular, a turbo molecular pump, which is a kind of vacuum pump, is widely used since it involves little residual gas and allows maintenance with ease, etc. Further, a semiconductor manufacturing process involves a number of steps of causing various process gasses to act on a semiconductor substrate, and the turbo molecular pump is used not only to create a vacuum in the chamber but also to evacuate such process gases from the chamber.
Further, in an equipment such as an electron microscope, a turbo molecular pump is used to create a high vacuum state within the chamber of the electron microscope, etc. in order to prevent refraction, etc. of the electron beam due to the presence of dust or the like.
Such a turbo molecular pump is composed of a turbo molecular pump main body 100 for sucking gas from the chamber of a semiconductor manufacturing apparatus or the like, and a control device 200 for controlling the turbo molecular pump main body 100.
FIG. 9 shows the construction of a turbo molecular pump.
In FIG. 9, the turbo molecular pump main body 100 has an inlet port 101 formed at the upper end of a round outer cylinder 127. On the inner side of the outer cylinder 127, there is provided a rotor 103 in the periphery of which there are formed radially and in a number of stages a plurality of rotary vanes 102a, 102b, 102c, . . . formed of turbine blades for sucking and evacuating gases. The rotor 103 is a substantially cylindrical member with a ceiling, and a rotor shaft 113 is passed for fixation through the center of the rotor 103 from the inner side thereof. The structure of the portion where the rotor shaft 113 and the rotor 103 are fixed to each other will be described in detail below.
Further, the rotor shaft 113 is supported in a levitating state and controlled in position by, for example, a so-called five-axis control magnetic bearing. A cylindrical main shaft portion 151 of the rotor shaft 113 is formed of a high magnetic permeability material (such as iron), and is attracted by the magnetic force of an upper radial electromagnet 104 and a lower radial electromagnet 105.
The upper radial electromagnet 104 includes four electromagnets arranged in pairs in the X-axis and the Y-axis. In close proximity to and in correspondence with the upper radial electromagnet 104, there is provided an upper radial sensor 107 composed of four electromagnets. Further, the upper radial sensor 107 detects a radial displacement of the main shaft portion 151 of the rotor shaft 113, and transmits a displacement signal to the control device 200.
In the control device 200, the upper radial electromagnet 104 is excitation-controlled through a compensation circuit with a PID adjustment function (not shown) based on the displacement signal obtained through detection by the upper radial sensor 107, thus adjusting the upper radial position of the main shaft portion 151 of the rotor shaft 113. Note that this adjustment is conducted independently in the X-axis direction and the Y-axis direction.
Further, the lower radial electromagnet 105 and a lower radial sensor 108 are arranged in the same way as the upper radial electromagnet 104 and the upper radial sensor 107, adjusting the lower radial position of the main shaft portion 151 of the rotor shaft 113 in the same manner as the upper radial position thereof.
Further, axial electromagnets 106A and 106B are arranged so as to sandwich from above and below a circular metal disc 111 provided in the lower portion of the main shaft portion 151 of the rotor shaft 113. The metal disc 111 is formed of a high magnetic-permeability material, such as iron.
Further, under the metal disc 111, there is provided an axial sensor 109 for detecting an axial displacement of the rotor shaft 113. An axial displacement signal obtained through detection by the axial sensor 109 is transmitted to the control device 200.
Based on the displacement signal obtained through detection by the axial sensor 109, the control device 200 excitation-controls the axial electromagnets 106A and 106B. At this time, the axial electromagnet 106A attracts the metal disc 111 upwardly by magnetic force, and the axial electromagnet 106B attracts the metal disc 111 downwardly.
In this way, the magnetic bearing appropriately adjusts the magnetic force applied to the rotor shaft 113, thereby magnetically levitating the rotor shaft 113 and retaining it in a non-contact fashion.
Further, there is provided a motor 121, which is equipped with a plurality of permanent magnet magnetic poles circumferentially arranged on the rotor side thereof so as to surround the main shaft portion 151 of the rotor shaft 113. A torque component rotating the rotor shaft 113 is applied to those permanent magnet magnetic poles from the electromagnets on the stator side of the motor 121, thereby rotating the rotor 103.
Further, the motor 121 is equipped with an RPM sensor and a motor temperature detecting sensor (not shown). The RPM of the rotor shaft 113 is controlled by the control device 200 on the basis of detection signals received from the RPM sensor and the motor temperature detecting sensor.
On the other hand, arranged on the rotor 103 to which the rotor shaft 113 is fixed are the rotary vanes 102a, 102b, 102c, . . . , in a number of stages as described above. Further, there are arranged a plurality of stationary vanes 123a, 123b, 123c, . . . , with a slight gap being between them and the rotary vanes 102a, 102b, 102c, . . . .
Further, in order to downwardly transfer the molecules of the exhaust gas through collision, the rotary vanes 102a, 102b, 102c, . . . are inclined by a predetermined angle with respect to planes perpendicular to the axis of the rotor shaft 113. In a similar fashion, the stationary vanes 123 are inclined by a predetermined angle with respect to planes perpendicular to the axis of the rotor shaft 113, and are arranged so as to protrude toward the interior of the outer cylinder 127 and in alternate stages with the rotary vanes 102.
Further, one ends of the stationary vanes 123 are supported while being inserted between a plurality of stationary vane spacers 125a, 125b, 125c, . . . stacked together. The stationary vane spacers 125 are ring-like members formed of a metal, such as aluminum, iron, stainless steel, or copper, or a metal such as an alloy containing those metals as the components.
Further, in the outer periphery of the stationary vane spacers 125, the outer cylinder 127 is provided with a slight gap therebetween. The outer cylinder 127 is fixed to a base portion 129 provided at the bottom thereof by bolts 128. Between the bottom of the stationary vane spacers 125 and the base portion 129, there is provided a threaded spacer 131. In the portion of the base portion 129 which is below the threaded spacer 131, there is formed an exhaust port 133, which communicates with the exterior.
The threaded spacer 131 is a cylindrical member formed of a metal, such as aluminum, copper, stainless steel, or iron, or a metal such as an alloy containing those metals as the components, and has on the inner peripheral surface thereof a plurality of spiral thread grooves 131a formed therein. The direction of the spiral thread grooves 131a is determined such that, when the molecules of the exhaust gas move in the rotating direction of the rotor 103, these molecules are transferred toward the exhaust port 133.
Further, in the lowermost portion of the rotor 103 connected to the blade-like rotary vanes 102a, 102b, 102c, . . . , there is provided the rotary vane 102d vertically downwards, which is formed in a cylindrical shape with respect to the axis of the rotor shaft 113. The rotary vane 102d protrudes toward the inner peripheral surface of the threaded spacer 131. This protruding part is placed in close proximity to the threaded spacer 131 with a predetermined gap therebetween.
Further, the base portion 129 is a disc-like member constituting the base portion of the turbo molecular pump main body 100, and is generally formed of a metal, such as iron, aluminum, or stainless steel. The base portion 129 physically retains the turbo molecular pump main body 100, and also functions as a heat conduction path, so it is desirable to use a metal that is rigid and of high heat conductivity, such as iron, aluminum, or copper, for the base portion 129.
When, with this construction, the rotor shaft 113 is driven by the motor 121 and rotates together with the rotor 103 and the rotary vanes 102, an exhaust gas from a chamber is sucked through the inlet port 101 by the action of the rotary vanes 102 and the stationary vanes 123.
Then, the exhaust gas sucked in through the inlet port 101 flows between the rotary vanes 102 and the stationary vanes 123 to be transferred to the base portion 129. At this time, the temperature of the rotary vanes 102 rises due to the friction heat generated when the exhaust gas comes into contact with the rotary vanes 102, conduction of the heat generated in the motor 121, etc, and this heat is transmitted to the stationary vanes 123 side by radiation or conduction due to the gas molecules, etc. of the exhaust gas. Further, the stationary vane spacers 125 are bonded together in the outer periphery, and transmit to the exterior the heat received by the stationary vanes 123 from the rotary vanes 102, the friction heat generated when the exhaust gas comes into contact with the stationary vanes 123, etc.
The exhaust gas transferred to the base portion 129 is sent to the exhaust port 133 while being guided by the thread grooves 131a of the threaded spacer 131.
In the above-described example, the threaded spacer 131 is provided in the outer periphery of the rotary vane 102d, and the thread grooves 131a are formed in the inner peripheral surface of the threaded spacer 131. However, conversely to the above, the thread grooves may be formed in the outer peripheral surfaces of the rotary vane 102d, and a spacer with a cylindrical inner peripheral surface may be arranged in the periphery thereof.
Further, in order that the gas sucked in through the inlet port 101 may not enter the electrical section formed of the motor 121, the lower radial electromagnet 105, the lower radial sensor 108, the upper radial electromagnet 104, the upper radial sensor 107, etc., the periphery of the electrical section is covered with a stator column 122, and a predetermined pressure is maintained in the interior of the electrical section with a purge gas.
For this purpose, piping (not shown) is arranged in the base portion 129, and the purge gas is introduced through the piping. The purge gas thus introduced flows through the gaps between a protective bearing 120 and the rotor shaft 113, between the rotor and stator of the motor 121, and between the stator column 122 and the rotary vanes 102 before being transmitted to the exhaust port 133.
Incidentally, for enhanced reactivity, the process gas may be introduced into the chamber in a high temperature state. When it reaches a certain temperature by being cooled at the time of evacuation, such process gas may be solidified to precipitate a product in the exhaust system. Then, when such process gas is cooled and solidified in the turbo molecular pump main body 100, it adheres to the inner portion of the turbo molecular pump main body 100 and is deposited thereon.
For example, when SiCl4 is used as the process gas in an Al etching apparatus, a solid product (e.g., AlCl3) is precipitated when the apparatus is in a low vacuum state (760 [torr] to 10−2[torr]) and at lower temperature (approximately 20[° C.]), and adheres to and is deposited on the inner portion of the turbo molecular pump main body 100 as can be seen from a vapor pressure curve.
When precipitate of the process gas is deposited on the inner portion of the turbo molecular pump main body 100, the deposit narrows the pump flow path, which leads to a deterioration in the performance of the turbo molecular pump main body 100. For example, the above-mentioned product is likely to solidify and adhere to the portion near the exhaust port where the temperature is low, in particular, near the rotary vanes 102 and the threaded spacer 131.
To solve this problem, there has been conventionally adopted a control system (hereinafter referred to as TMS; temperature management system), in which a heater (not shown) and an annular water cooling tube 149 are wound around the outer periphery of the base portion 129 or the like, and in which a temperature sensor (e.g., a thermistor) (not shown) is embedded, for example, in the base portion 129, the heating by the heater and the cooling by the water cooling tube 149 being controlled based on a signal from the temperature sensor so as to maintain the base portion 129 at a fixed, high temperature (set temperature).
Here, the conventional structure of the portion where the rotor shaft 113 and the rotor 103 are fixed to each other will be described. FIG. 10 is an enlarged structural view of the portion where the rotor shaft and the rotor are fixed to each other, FIG. 11 is a partial structural view of the rotor, and FIG. 12 is a partial structural view of the rotor shaft. FIG. 12(a) is a longitudinal sectional view of the rotor shaft, and FIG. 12(b) is a plan view of the same.
As shown in FIGS. 10 through 12, in the rotor shaft 113, on top of the main shaft portion 151 whose radial position is adjusted by the above-mentioned upper radial electromagnet 104, etc., there is formed a fastening portion 153 whose diameter is increased step wise up to approximately double the diameter of the main shaft portion 151. Over the entire upper surface of the fastening portion 153, there is formed a rotor shaft 113 side contact surface 157 to be brought into contact with the rotor 103, and the contact surface 157 is machined so as to be perpendicular to the axial direction of the main shaft portion 151 and as to be flat.
Further, in the fastening portion 153, there are formed bolt holes 161 open on the contact surface 157 side and extending in an axial direction, and the bolt holes 161 are formed at positions spaced apart from the axis of the rotor shaft 113 by a distance substantially the same as the radius of the main shaft portion 151. Further, the bolt holes 161 are formed, for example, at six positions in the fastening portion 153, and arranged at equal intervals around the axis. The number of the bolt holes 161 is not restricted to six; it may also be, for example, eight.
Further, extending upwardly from the fastening portion 153 of the rotor shaft 113 is a pass-through shaft portion 155 whose diameter is smaller than that of the main shaft portion 151 and whose axis is matched with that of the main shaft portion 151. Further, in the upper end portion of the pass-through shaft portion 155, there is formed a hexagonal hole 163 upwardly open and extending in an axial direction. The hexagonal hole 163 extends to a depth corresponding to approximately half the length of the pass-through shaft portion 155.
On the other hand, in the central portion of the upper end of the rotor 103, there is formed a downwardly extending recess 181 with around sectional configuration. At the center of the recess 181, there is formed a central hole 183 axially extending between the inner side and the outer side of the rotor 103.
Further, below the recess 181 and on the surface on the inner side of the rotor 103, there is formed a rotor 103 side contact surface 187 to be brought into contact with the contact surface 157 of the rotor shaft 113. The contact surface 187 is also machined so as to be perpendicular to the axial direction.
Further, in the recess 181, there are formed bolt passing holes 185 adjacent to the central hole 183 and extending axially between the inner side and the outer side of the rotor 103. The bolt passing holes 185 are formed in the same number as the bolt holes 161 on the rotor shaft 113 side, and are arranged so as to communicate with the bolt holes 161 when the pass-through shaft portion 155 of the rotor shaft 113 is passed through the central hole 183 of the rotor 103.
Further, in the state in which the bolt passing holes 185 communicate with the bolt holes 161, the leg portions of bolts 191 are passed through the bolt passing holes 185; further, the bolts 191 are threadedly engaged with the bolt holes 161 on the rotor shaft 113 side. The bolts 191 are also prepared in the same number as the bolt holes 161.
With this construction, when fixing the rotor shaft 113 and the rotor 103 to each other, the pass-through shaft portion 155 of the rotor shaft 113 is first inserted into the central hole 183 of the rotor 103. At this time, the insertion of the pass-through shaft portion 155 into the central hole 183 is effected, for example, by shrinkage fit.
Thus, at room temperature, the outer diameter of the pass-through shaft portion 155 of the rotor shaft 113 is larger than the inner diameter of the central hole 183 of the rotor 103 by approximately several tens of μm. Prior to the insertion of the pass-through shaft portion 155, solely the rotor 103 is heated to approximately 100° C., and the inner diameter of the central hole 183 of the rotor 103 is made larger than the outer diameter of the pass-through shaft portion 155 of the rotor shaft 113 by approximately several hundreds of μm. After this, the pass-through shaft portion 155 is inserted into the central hole 183 in this state, and left to stand for a fixed period of time for cooling. As a result, when the rotor 103 and the rotor shaft 113 are restored to room temperature, the pass-through shaft portion 155 is firmly fixed to the central hole 183 due to the difference in diameter at room temperature.
After the cooling of the rotor 103 and the rotor shaft 113 fixed to each other by shrinkage fit, the bolts 191 are threadedly engaged with the bolt holes 161 on the rotor shaft 113 side. In fastening the bolts 191, a hexagonal wrench (not shown) is fittingly engaged with the hexagonal hole 163 of the rotor shaft 113, thereby preventing rotation of the rotor 103 and the rotor shaft 113. As a result, the rotor 103 and the rotor shaft 113 are easily fastened together.